Flexible ‘skin’ helps prostheses users sense sheer force

Researchers from the University of Washington and the University of California Los Angeles have developed a flexible sensor that can be stretched over a prostheses as a “skin,” and accurately sends information regarding sheer forces and vibration, according to a press release.

Researchers, who published their findings in Sensors and Actuators A: Physical, reported the skin could allow prostheses users to sense when an object is slipping out of their grasp. Such sensations could help users to better manipulate objects and perform everyday tasks, they added.

“Robotic and prosthetic hands are based on visual cues right now — such as, ‘Can I see my hand wrapped around this object?’ or ‘Is it touching this wire?’” Jonathan D. Posner, PhD, co-author of the paper and professor of mechanical engineering and of chemical engineering at the University of Washington, said in the release. “But that’s obviously incomplete information.”

According to the researchers, the skin mimics the way a human finger responds to tension and compression as it slides along a surface or distinguishes among different textures. Manufactured from the same silicone rubber used in swimming goggles, the skin is embedded with tiny channels that are approximately half the width of a human hair, which are filled with electrically conductive liquid metal. According to the release, unlike solid wires, the liquid metal resists cracks and fatigue when the skin is stretched.

When the skin is placed around a prosthetic finger, these channels are strategically placed on either side of where a human fingernail would be. As the user slides their prosthetic finger across a surface, the channels on one side of the nailbed compress while the ones on the other side stretch out, similar to a natural limb.

As the channel geometry changes, so does the amount of electricity. According to release, researchers can measure these differences in electrical resistance and correlate these with shear forces and vibrations that the prosthetic finger is experiencing.

According to the researchers, the skin possesses a “high level of precision and sensitivity for light touch applications,” such as opening a door, using a phone, shaking hands, lifting packages and handling objects. In addition, experiments have shown that the skin can detect tiny vibrations at 800 times per second — better than human fingers.

“By mimicking human physiology in a flexible electronic skin, we have achieved a level of sensitivity and precision that’s consistent with human hands, which is an important breakthrough,” Posner said in the release. “The sense of touch is critical for both prosthetic and robotic applications, and that’s what we’re ultimately creating.”